FR2569130A1 - METHOD AND DEVICE FOR CONTINUOUS CRYSTALLIZATION, IN PARTICULAR SUCROSE - Google Patents

Abstract

1. Method of continuous-process crystallization, in which a mixture of crystals surrounded by mother liquor is urged along a process path (C1, C2) through a reactor, feeding liquor is supplied to the mixture throughout the process path, said feeding liquor providing the nourishment required for crystal growth, and the mixture is brought to a boil with the help of a heating surface in order to bring about a concentration of the mixture diluted by the feeding liquor, characterized in that stirring of the mixture is intensified along the process path (C1, C2) in order to keep the specific rate of crystallization to a substantially constant optimum value over most of the process path (C1, C2), said intensifying of the stirring being obtained by modifying the mixture boiling rate by an increase of the heating surface (15) along the process path (C1, C2), said surface increase as well as the supply of matter following an exponential distribution as a function of the length of the process path inside the reactor, the exponent preferably ranging between 0.625 and 0.75.

Description

1 2569130

  Process and device for continuous crystallization, especially sucrose The invention relates to continuous crystallization, and

  mainly the crystallization of sucrose.

  The continuous crystallization of sucrose takes place in crystallizer-evaporators in which the magnification of crystals, coming from outside, or produced in the crystallizer by controlled nucleation is carried out. These crystals are nourished, throughout their course, by an aqueous liquor sweetened, concentrated but under-saturated, which is supersaturated by

  evaporation inside the device.

  When the nucleation is not carried out in the apparatus itself, it is carried out in a separate apparatus, and one obtains a mass-cooked "foot", containing the small crystals which one

  will grow in the continuous crystallizer.

  It is recalled that the term "mass-cooked" in the sugar terminology is the mixture constituted by the crystals (phase

  solid) and the supersaturated mother liquor surrounding them (liquid phase).

  It is also possible to produce a "foot" with small crystals which are mixed with a supersaturated liquor, the

mixture being referred to as "magma".

  Continuous crystallizers are generally horizontal reactors, compartmentalized or not, containing a heat exchanger, fed with steam, in order to provoke

the boiling of the mass-cooked.

  Such compartmentalised reactors are known for example

  in French Patent Nos. 1581088 and 2154898.

  Non-compartmentalized continuous reactors are known for example from French patents Nos. 2,144,945 and 2,305,496.

in the name of the Applicant.

  The boiling obtained at low temperature thanks to the vacuum (or more exactly to subatmospheric pressures) prevailing in the apparatus, is necessary not to degrade the sugars and organic impurities that accompany them. It allows to concentrate the mass-cooked, diluted by the nourishing solution which

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  is provided throughout the course.

  Thus the crystallization operation, in sugar refinery or refinery, has a dual purpose. It makes it possible to separate sucrose from a solution containing, in addition to sucrose, other soluble products called "impurities". This operation makes it possible to obtain much purer crystals than the solution which served to make them fat, so it is a purification operation. On the other hand, it makes it possible to obtain sucrose in solid phase, easy to preserve and to condition. For technical and commercial reasons, it is important to produce crystals of regular size. The object of the invention is to improve the known processes and continuous crystallization device, in particular under the

  ratio of the crystallization yield, and therefore of the production.

  To achieve this goal, the invention proposes a method of the type according to which progress is made along a path of

  treatment in a reactor a mixture of crystals surrounded by water-

  mother, it feeds the feed liquor mixture as it progresses in the reactor, the liquor being intended to provide the necessary contribution to the magnification of the crystals, and the boiling of the mixture, with the aid of a heating surface, in order to obtain the concentration of the mixture diluted by the feeder liquor. In accordance with the invention, stirring of the mixture is intensified along the treatment path so as to maintain the specific crystallization rate at an optimum constant value over most of the path. Indeed, as will be explained below, the researches and experiments carried out by the Applicant have shown that in the known processes, the progression of the crystallization is accompanied by an increase in the viscosity of the mass-cooked braking brewing leading, in the absence of any palliative, to an unfavorable evolution of the

  crystallization rate along the path.

  According to an advantageous original aspect of the invention, the stirring is intensified by modifying the boiling regime of the mixture 3 -o2569130. The various means will be detailed below.

particulars of this general means.

  Although all the phenomena governing the conduct of a crystallization reactor are particularly complex, and that the Applicant does not intend to be bound by the scientific considerations that will develop, it seems that the interest of

  the invention is explained at least in part as follows.

  The specific rate of crystallization T is the amount of material (sucrose) deposited, per unit of time, on the surface unit of a crystal, It depends on four main factors: 1. The purity of the nutrient solution. The treated solutions always contain soluble mineral or organic materials which hinder the crystallization of sucrose. The T rate

  decrease with increasing percentage of impurities contained.

  2. The supersaturation of the mother water. It must be maintained at its highest value in the "metastable" zone (where crystals grow but new crystals do not form), that is, as close as possible to the "labile" zone ( o new crystals are formed) but without reaching it so as not to risk the formation of "false grains". The

  T rate is a direct function of the supersaturation.

  3. The temperature. The rate T is a direct function of the temperature. The fact of passing from 70 to 80 C doubles the value of T. But it is not necessary to go above 90 C in practice not to destroy the products in manufacturing, organic, so

sensitive to heat.

  4. The brewing of the mass-cooked. When the massecuite is immobile, the sucrose molecules approach the surface of the crystals, on which they will be fixed, solely by a diffusion phenomenon. Since the mother liquor is very viscous, this displacement is slow and limits the rate T. A mixing of the massecuite makes it possible to permanently contact crystals of new sucrose molecules and increase the value of the rate, which is almost

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proportional to the brewing.

  There is therefore a compromise between the supersaturation, which causes an increase in viscosity and slows the stirring, and the latter, at a certain temperature, to reach the

  optimum value of the crystallization rate.

  The mass of product deposited on a crystal over time

  is equal to the product of its surface by the rate of crystallization.

  To obtain the best performance of a crystallizer, it is necessary to optimize the rate and always maintain the largest surface of

  fixation possible, that is to say as many crystals as possible.

  One might be tempted to fill the reactor with as many very small crystals as possible, leading to a larger area than large, small crystals. Nevertheless, commercial constraints and, in certain technical cases, cause the final size of the crystals to always be determined and the magnification to be limited, for the same reasons to a size ratio of between 1.4 and 3.

(ratio of crystal lengths).

  A well conducted continuous crystallizer must therefore contain, within the mass-cooked it contains, the maximum of crystals,

  from the beginning to the end of the course.

  The Applicant has discovered that this amount of crystals,

  "percentage" or "yield" of crystals, mass

  cooked, causes a compactness (viscosity) increasing, which makes that the brewing of the mass-cooked is decreased in unacceptable proportions to maintain the rate of crystallization at its best value (to note that at the end of the course one can and even the yield of crystals must be allowed to increase to the maximum in order to exhaust as much as possible the water of the massecuite,

for the so-called tightening operation).

  It results from studies that the viscosity substantially follows a logarithmic law as a function of the length of the path in the reactor, a law marked by a sharp increase in the slope from a certain length. Conversely, the coefficient of heat transfer is therefore falling quite

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  brutal at the same time. This variation, depending on the reactor line, can easily cause the heat transfer coefficient to drop to 10% or even 1% at the end of the run.

what it is at the beginning.

  This drop in the coefficient of heat transfer causes, in the absence of remedies, the virtual impossibility of

  maintain the crystallisation rate at its optimum value,

  that is, the value of the material feed to be crystallized is exactly adjusted to the growth potential of the surface of the

crystals.

  The invention overcomes the drop in the heat transfer coefficient by a gradual intensification of the stirring along the path, during the crystallization process. This intensification of the brewing is preferably done

  by intensifying the evaporation power along the path.

  In known apparatuses, the evaporation power is substantially constant, by construction, on the crystallization path (that is, with the possible exception of at all

  beginning of the course or the end of the course).

  According to the invention, the boiling regime of the mixture can be modified by several means used alone or in combination. According to a first solution, the mixture is fed with a feeder liquor of concentration (Brix) decreasing on

along the way.

  The concentration variation is made in stages, corresponding to the compartments, or groups of compartmentalized compartments, and to areas of constant length

  or variable for non-compartmentalized devices.

  The possible disadvantage of this solution is the extra energy consumption it entails. Indeed, we normally try to feed the crystallizers with solutions as concentrated as possible, this concentration being carried out in multiple-effect evaporators, economic

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  in energy. Partially diluting these solutions and reconcentrating them again, as a single effect in the crystallizer, results in an additional energy consumption which is acceptable in candy cane where there is a lot of free energy, but can become undesirable in candy

beet and refinery.

  According to a second solution, the boiling regime is modified by increasing the heating intensity along the path. Accordingly, this solution consists in splitting the total heating surface into a number of independent elements in each of which the flow of steam just needed for the compartments or areas they serve. There may be an independent element per compartment, or compartment group for compartmentalized appliances, or areas of equal or unequal length for non-compartmentalized reactors. This solution has the advantage of not involving additional energy expenditure because it is possible to use a nourishing solution of constant concentration. On the other hand, it can lead to a higher construction cost because the total heating surface must be split into several separate elements and fed individually, which requires piping and heating.

  additional regulation equipment.

  According to a third solution, judged to be the most advantageous, the boiling regime is modified by an increase in the

  heating surface, along the crystallization path.

  This solution therefore consists in modulating, by construction, the heating surface, along the path of the massecuite, so that, taking into account the variation of the heat transfer coefficient, the apparatus is supplied with now substantially constant the yield of crystals, on

  most of the course of the mass-cooked.

  This modulation is done for each compartment or group of compartments for compartmentalised devices and for

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  successive areas or in a completely continuous manner in the

non-compartmentalized apparatus.

  This modulation can be obtained: by modifying the profile and / or the number of the heating plates if they are elements of this type which constitute

1 heat exchanger.

  - by changing the diameter of the tubes, the length and the number of horizontal heating pins for certain appliances. - by changing the height of the tubular bundle

  vertical heating, progressive or phased.

  by modifying the pitch and / or the diameter of the tubes constituting the tubular bundle (that is to say a

  modification of the tube section density).

  The modulation of the heating surface may be the result of one or a combination of several of these measurements. This solution, especially in its last variant, is very easy to realize and concretizes the goal without

  cause additional cost due to additional equipment.

  The ratio of heating surface to volume of mass-cooked must therefore be an increasing function of the travel time of the massecuite within the reactor. The increase of this ratio between the beginning of crystallization course and the end can be in a proportion of 1 to 3. The heating can be still

  increased in the very last step for tightening the mass-

  cooked. Finally, in order to improve the heat transfer coefficient when working with compact products, it may be advantageous to promote mixing by installing

  mechanical devices intended to force the circulation of mass

  cooked through the heating elements and thereby increase

  the efficiency of heat exchange.

  The equipment used may be pumps or propellers installed outdoors, or preferably indoors. 8 r2 69130

of the device.

  The power used by these circulators is compensated

  by the saving of the heating surface that it entails.

  The invention also relates to a continuous crystallization apparatus, of the compartmentalized or non-compartmentalized type, intended

  to the implementation of the crystallization process presented below

  above. The apparatus of the invention comprises means for intensifying the stirring of the mixture along the path of the massecuite. These means, which advantageously control the boiling point of the mixture, may comprise, individually or in combination: heating means with increasing surface along the path, heating means with increasing intensity along the path; nourishing liquor supply decreasing concentration along the route,

mechanical stirrers.

  Other features and advantages of the invention

  from the following description given by way of example

non-limiting.

  Reference is made to the accompanying drawings in which: - Figure 1 is a vertical radial section I-I of a circular reactor of the invention, Figure 2 is a broken section II-II of the reactor of

Figure 1.

  There is shown in the drawings a non-compartmental circular continuous reactor. The basic principle of such a reactor is that which has been developed in the document

  FR-A-2305496 to which reference will be made.

  However, while the description of the aforementioned document

  more particularly - but not exclusively - a straight reactor, it has been found preferable for reasons of space and construction to adopt a reactor 9 d256913O circular. The foot or magma enters through the inlet 1, first runs through the external DC circuit C1, in the direction of the arrows 2, to the radial wall 3; the mass-pass through the spillway 4 in the internal continuous circuit C2 that it travels in the direction of the arrows 5, until meeting the radial partition 3 and go through the spillway 6 in the central compartment C3. The terracotta escapes through the

weir 12.

  The circuits and the central compartment are delimited by an outer shell 7, inner ferrules 8 and 9,

  tapered bottoms 10 and tapered lids 11.

  The tubular heating bundles are formed by double insulating walls 13, the tube-carrying plates 14 and the tubes 15 of which only a few are shown. The heating vapor enters the beams through the arrivals 16, and the

  condensates escape through the exits 17.

  According to the aforementioned document FR-A-2 305 496, radial baffles 18 and baffles or conical caps 19 are provided to prevent disturbances related to a disordered boiling of the massecuite. The vapors circulate above the compartments (which can pass from compartment C3 to compartments C2-C1 through openings 20 provided in shell 9) and then

  escape through the outlet 21 to a barometric condenser 23.

  The feeding of the circuits C1 and C2 in the feeder liquor is carried out at the base of the tubular bundles by means of the collectors 24 and 25, the metering tubes 26, the pipes 27 and the isolation valves 28. A channel is provided every 6, or 30 pipes for the

  circuit C1 and 30 pipes for the circuit C2.

  The compartment C3 comprises a mechanical circulator 29

mass-cooked.

  For the implementation of the modulation of the heating surface each of the circuits C1 and C2 is divided into six consecutive sectors of 600. The number of the tubes 15 varies in the

2

  sectors, according to the invention. For example, for a reactor of 6, 75m in diameter, there are provided in the first 6 ring sectors of the first circuit (internal diameter 5240mm and external 6280mm) 57.67,77.83, 88.92 tubes of 1Ocm diameter; the

  no tubes goes naturally decreasing with the sectors.

  For the following six sectors of the circuit C2 (internal diameter 3820mm, and external 5240mm) there are 92,98,104,104,108 respectively, 108 tubes of the same diameter. The beam of the central compartment (internal diameter torus 1200 mm and external 2996 mm) counts 331

tubes of the same diameter.

  At the modulation of the heating surface, the difference in the concentration of the mother liquor has been combined since, as shown in FIG. 1, the collector 25 is fed with

  feeder liquor diluted with respect to the collector 24.

  Naturally, this decreasing concentration can be done

  sector by sector, or by sector group.

  The tubular bundles may also be compartmentalised by radial partitions 30, corresponding or not to the toric sectors previously defined and selectively fed with steam so as to modulate the intensity of the heating.

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Claims (13)

  1. Continuous crystallization process, according to which a mixture of crystals surrounded with mother liquor is progressed along a treatment path (C1, C2) in a reactor, the mixture is fed into the feed liquor as and when as it progresses in the reactor, this liquor being intended to supply the necessary contribution to the magnification of the crystals, and the mixture is boiled with the aid of a heating surface in order to obtain the concentration of the mixture diluted by the feeder liquor, characterized in that the stirring of the mixture is intensified along the treatment path (C1, C2) so as to maintain the specific degree of crystallization substantially at a value   optimum constant over most of the path (C1, C2).   2. Method according to claim 1, characterized in that intensifies the stirring by changing the boiling regime of the mixture.   3. Method according to claim 2, characterized in that said diet is modified by an increase in the area of   heating (15) along the path (C1, C2).   4. Method according to any one of claims 2   or 3, characterized in that said regime is modified by an increase in the intensity of heating, along the path (C1, C2).   5. Method according to any one of claims 2 to   4, characterized in that said diet is modified by feeding the mixture with a liquor of decreasing concentration, along the path (C1, C2).   6. Continuous crystallization device of the type comprising a reactor defining a treatment path (C1, C2)   for a mixture of crystals surrounded by mother liquor, means (24-   28) feeding the feed liquor mixture along the path (C1, C2) a heating surface (15) for causing the mixture to boil, characterized in that it comprises, arranged along the path 12 2569130   (Cl, C2) means for intensifying mixing of the mixture, so as to maintain the specific crystallization rate at an optimum constant value over the largest part of the path (C1, C2).   7. Device according to claim 6, characterized in that said intensification means comprise mechanical stirrers.   8. Device according to any one of the claims   6 or 7, characterized in that said intensifying means   act on the boiling regime of the mixture.   9. Device according to claim 8, characterized in that said means are constituted by the heating surface   (15) increasing along the path (C1, C2).   10. Device according to claim 9, characterized in that the heating surface consists of bundles of tubes.   (15) of increasing density along the path (C1, C2).   11. Device according to any one of the claims   8 to 10, characterized in that said means are constituted by means for increasing the intensity of heating along of the path (C1, C2).   12. Device according to any one of the claims   8 to 11, characterized in that said means are constituted by means for feeding the mixture with a liquor of   decreasing concentration along the path (C1, C2).   13. Device according to any one of the claims   8 to 12, characterized in that the reactor is circular.